Codechange: Improve performance of evaluating group hierarchy.

Store a list of child groups in each group, to avoid having to iterate the group pool to find its children.

src/saveload/afterload.cpp

@@ -575,6 +575,9 @@ bool AfterLoadGame()
 	 * that otherwise won't exist in the tree. */
 	RebuildViewportKdtree();
 
+	/* Group hierarchy may be evaluated during conversion, so ensure its correct early on. */
+	UpdateGroupChildren();
+
 	if (IsSavegameVersionBefore(SLV_98)) _gamelog.GRFAddList(_grfconfig);
 
 	if (IsSavegameVersionBefore(SLV_119)) {